Human immunodeficiency virus (HIV) damages the immune system, causing a reduction in lymphocyte numbers. A diagnosis of AIDS may be made once lymphocyte numbers fall below $200$ cells mm$^{-3}$ of blood. The graph shows the lymphocyte numbers after HIV infection in a patient. After how many years from infection would this patient be diagnosed with AIDS?
- A$0$ years
- B$3$ years
- C$7$ years
- D$9$ years